HCV is now recognized as the major agent of chronic hepatitis and liver disease worldwide. It is estimated that HCV infects about 400 million people worldwide, corresponding to more than 3% of the world population.
Hepatitis C virus (“HCV”) is a small enveloped RNA flavivirus, which contains a positive-stranded RNA genome of about 10 kilobases. The genome has a single uninterrupted ORF that encodes a protein of 3010–3011 amino acids. The structural proteins of HCV include a core protein (C), which is highly immunogenic, as well as two envelope proteins (E1 and E2), which likely form a heterodimer in vivo, and non-structural proteins NS2–NS5. It is known that the NS3 region of the virus is important for post-translational processing of the polyprotein into individual proteins, and the NS5 region encodes an RNA-dependant RNA polymerase.
Virus-specific T lymphocytes, along with neutralizing antibodies, are the mainstay of the antiviral immune defense in established viral infections. Whereas CD8+ cytotoxic T cells eliminate virus-infected-cells, CD4+ T helper cells are essential for the efficient regulation of the antiviral immune response. CD4+ T helper cells recognize specific antigens as peptides bound to autologous HLA class II molecules (viral antigens or particles are taken up by professional antigen-presenting cells, processed to peptides, bound to HLA class II molecules in the lysosomal compartment, and transported back to the cell surface). Several observations support an important role of CD4+ T cells in the elimination of HCV infection. Tsai et al., 1997 Hepatology 25:449–458; Diepolder et al 1995 Lancet 346:1006–1007; Missale et al 1996 JCI 98: 706–714; Botarelli et al 1993; Gastro 104: 580–587; Diepolder et al 1997 J. Virol 71: 6011. Immunogenic peptides usually have a minimal length of 8–11 amino acids. However, since the peptide binding groove of HLA class II molecules seems to be open at both ends, longer peptides are tolerated. Thus peptides eluted from HLA class II molecules are typically in the range of 15–25 amino acids. HLA class II molecules are extremely polymorphic and each allele seems to have its individual requirements for peptide binding. Thus the HLA class II repertoire of a given individual determines which viral peptides can be presented to T cells. Recognition of the specific HLA-peptide complex by the T cell receptor accompanied by appropriate costimulatory signals lead to T cell activation, secretion of cytokines, and T cell proliferation.
Numerous studies demonstrate that HLA Class II restricted CD4+ responses are determined by stimulating peripheral blood mononuclear cells with recombinant viral antigens or peptides. Botarelli et al., (1993) Gastroenterology 104:580–587; Farrari et al., (1994) Hepatology 19:286–295; Minutello et al., (1993) C. J. Exp. Med. 178:17–25; Hoffmann et al., (1995) Hepatology 21:632–638; Iwata et al., (1995) Hepatology 22:1057–1064; and Tsai.et al., (1995) Hepatology 21:908–912.
Polyclonal multispecific CD8+ T cell responses have been detected in patients with chronic hepatitis C. Additionally, CD8+ CTL's were shown to be important in resolving acute HCV infection in chimpanzees (Cooper et al., Immunity 1999). About 50% of patients with chronic hepatitis C demonstrate a detectable virus-specific CD4+ T cell response, which is most frequently directed against HCV core and/or NS4 and tends to be more common in patients who achieve sustained viral clearance during interferon-α therapy.
Depending on the pattern of lymphokines, CD4+ T helper cells have been classified as TH1, TH0, or TH2. Cytokines of the TH1 type are typically IFN-γ, lymphotoxin, and interleukin-2 (IL-2), which are believed to support activation of virus-specific CD8+ T cells and natural killer cells. The TH2 cytokines IL-4, IL-5, IL-10, and IL-13 are important for B cell activation and differentiation, thus inducing a humoral immune response.
During acute hepatitis C infection a strong and sustained TH1/TH0 response to NS3 and possibly to other nonstructural proteins is associated with a self-limited course of the disease. Diapolder et al., (1995) Lancet 346:1006–1007, showed all CD4+ T cell clones to have a TH1 or TH0 cytokine profile, suggesting that the clones support cytotoxic immune mechanisms in vivo. The majority of CD4+ T cell clones responded to a relatively short segment of NS3, namely amino acids 1207–1278, suggesting that this region of NS3 is immunodominant for CD4+ T cells. More than 70% of those who contract HCV develop chronic infection and hepatitis, and a significant portion of them progress to cirrhosis and eventually hepatocellular carcinoma. The only approved therapy at present is a 6- to 12-month course of interferon α, which leads to sustained improvement in only 20% of patients. So far, no commercial vaccine is available.
Thus, there remains a need for compositions and methods capable of promoting anti-HCV responses.